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Private Water Supply Zones Feasibility Project DWI 70/2/318 Private Water Supply Zones Feasibility Project DWI 70/2/318 Technical Report Defra October 2018 Notice This document and its contents have been prepared and are intended solely as information for Defra in relation to the Private Water Supply Zones Feasibility Project DWI 70/2/318. Atkins Limited assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents. This document has 56 pages including the cover, plus 24 pages in Appendix A and 27 pages in Appendix B. Document history Purpose Revision description Originated Checked Reviewed Authorised Date 1.0 Draft for client RD, MS HS DR DR 31/08/18 comment 2.0 Draft for client RD, MS HS DR DR 05/09/18 comment 3.0 Updated RD HS DR DR 26/10/18 following client comment received 15/10/18 Contents Chapter Page Executive Summary 4 1. Introduction 5 1.1. Background 5 1.2. Objectives 5 1.3. Report structure 6 2. Data collation and review 7 3. Selection of the two trial areas 11 4. Approach to defining conceptual zones and application of this method to the trial areas 12 4.1. Overview 12 Contains sensitive information 5162737/8/DG/010 | 3.0 | October 2018 Atkins | pws zones feasibility report v3.0 Page 2 of 56 4.2. Groundwater conceptual areas 12 4.3. Surface water conceptual areas 27 4.4. Catchment counts 28 4.5. Contamination sources 29 5. Validation of conceptual zones using historical water quality data 29 5.1. PrWS water quality data for England and Wales 29 5.2. Validation approach for two trial areas 33 5.3. Statistical significance test 37 5.4. Conclusions of validation of conceptual zones 43 5.5. Radioactivity 43 6. Potential cost savings from undertaking reduced PrWS monitoring 49 6.1. Approach to estimating cost savings 49 6.2. Estimate cost savings 50 7. Conclusions and recommendations 51 7.1. Conclusions 51 7.2. Recommendations 53 8. References 54 Appendix A. Trial area selection – technical methodology 56 Appendix B. Defining conceptual areas – technical methodology 47 7 Appendix C. Summary of cost estimates for laboratory water quality analysis 48 Appendix D. List of water quality determinands 51 Tables Table 2-1 – Summary of data collated 8 Table 4-1 – Sum of conceptual catchments by type 23 Table 5-1 - Summary count of determinands, sample points and results by trial LA 24 Table 5-2 - Count of excluded sample points by trial LA 24 Table 5-3 - Count of sample points by conceptual zones for the simplified approach 25 Table 5-4 - Count of sample points by conceptual zones for the detailed approach 26 Table 5-5 - Results of Kruskal-Wallis tests for Conwy 31 Table 5-6 - Results of Kruskal-Wallis tests for West Dorset 32 Table 5-7 - Number of sample points with five or more results for Conwy conceptual zones 33 Table 5-8 - Number of sample points with five or more results for West Dorset conceptual zones 34 Table 6-1 - Annual sampling rate for determinands where a reduction in sampling might be justified for Conwy 39 Table 6-2 - Annual sampling rate for determinands where a reduction in sampling might be justified for West Dorset 40 Table 6-3 - Annual sampling saving for Conwy 40 Table 6-4 - Annual sampling saving for West Dorset 40 Figures Figure 4-1 – Schematic of theoretical groundwater conceptual zones 13 Figure 4-2 – WFD Operational Groundwater Catchments in: a) Conwy, b) West Dorset 15 Figure 4-3 - Indicative geological cross section for West Dorset showing simplified approach to Contains sensitive information 5162737/8/DG/010 | 3.0 | October 2018 Atkins | pws zones feasibility report v3.0 Page 3 of 56 groundwater conceptual zones 16 Figure 4-4 – Defining detailed groundwater conceptual areas for bedrock geology 17 Figure 4-5 - Indicative geological cross section for West Dorset showing detailed approach to groundwater conceptual zones 19 Figure 4-6 - Defining groundwater conceptual areas for superficial geology 20 Figure 4-7 – Surface water conceptual areas in Conwy 21 Figure 4-8 – Surface water conceptual areas in West Dorset 22 Figure 5-1 - Timeseries data for conductivity across the conceptual zones in Conwy 29 Figure 5-2 – Example box and whisker plot for iron in Conwy 34 Figure 5-3 - Example box and whisker plot for iron in West Dorset 35 Figure 5-4 – Monitoring for radioactivity in public water supplies in West Dorset 37 Figure 5-5 – Monitoring for radioactivity in public water supplies in Conwy 38 Executive Summary Local Authorities (LAs) in England and Wales have a legal requirement to monitor Private Water Supplies (PrWS) to protect public health. In 2016 LA records contained details of 36,565 PrWS in England, and 14,981 in Wales (DWI, 2017a, 2017b). From 2017, amendments to the Drinking Water Directive have provided LAs an opportunity to monitor drinking water parameters in PrWS using a more flexible risk-based approach. This project seeks to develop these approaches further. The objective of this project was to investigate whether it is feasible to group PrWS together to reduce monitoring by sampling from a source which is representative of the water quality across a defined area. Limitations and risks of this approach along with potential cost savings have also been identified as part of this project. Grouping criteria for PrWS were developed separately for surface and groundwater sources using delineations such as Water Framework Directive water bodies, bedrock geology and aquifer vulnerability. Criteria were grouped in two different ways, one simpler and the other more complex. The homogeneity of water quality within these zones, and differences between them, were then assessed using historical water quality data from PrWS in two trial LAs: Conwy and West Dorset. Historical water quality data were interrogated using a number of methods to determine whether the source water quality was consistent for the conceptual zones. Summary statistics for the conceptual zones and determinands were produced but were not very useful in proving or disproving the hypothesis that the zones were consistent. Kruskal-Wallis tests for differences were used to determine whether the results from sample points within conceptual zones came from different distributions, however, it was found that there were not enough data per determinand per sample point to complete enough analyses over the LAs in order validate the conceptual zoning method. For a limited number of conceptual zones and determinands evidence of homogeneity was found. These zones and determinands were taken forward to assess whether sampling rates could be reduced. If all the results for a conceptual zone and determinands were below 60% of the Prescribed Concentration or Value (PCV) then the number of sample points and the annual sampling rate was analysed. Based on this statistical approach, and taking account of risk, the annual cost savings by reducing sampling and/or individual analyses were found to be are negligible. Results indicated that savings would only be achieved in the laboratory as the sample points would still need to be visited to collect samples for other determinands. While it would be possible to either modify the existing method to increase potential savings or even to develop a different method, it is unlikely that a statistically robust method could be developed that will make significant savings across the two trial LAs. Contains sensitive information 5162737/8/DG/010 | 3.0 | October 2018 Atkins | pws zones feasibility report v3.0 Page 4 of 56 If further investigation of conceptual zoning approaches is to be followed up, it is recommended: • The feasibility of defining zones is reassessed in 4 or 5 years’ time when more data are available. • That LAs with a higher density of PrWS are investigated to ensure that lack of data is less of an issue and that there is a greater probability of identifying potential savings. • The method applied here excludes some of the historical water quality data for reasons including quality assurance and uncertainty over the source of the water. Undertaking some further work in liaison with the LA could resolve some of these issues and would allow more data to be used in the assessment. • Consideration is given to alternative methods for PrWS zone definition; in particular, datadriven approaches as opposed to the conceptually driven approach that was the brief for this project. 1. Introduction 1.1. Background Under the Private Water Supplies (England) Regulations 2016, the Private Water Supplies (England) (Amendment) Regulations 2018 and the Private Water Supplies (Wales) Regulations 2017 which transpose the Drinking Water Directive and Euratom Directive, there is a legal requirement for Local Authorities (LAs) to monitor private water supplies (PrWS). The Regulations set standards for drinking water and the aim is to protect public health from the adverse effect of any contamination by ensuring water for human consumption is wholesome and clean. Depending on the Regulation classification of each PrWS up to a total of 48 microbiological, chemical and indicator parameters must be monitored and tested regularly. The Directives allow for monitoring frequencies to apply, based on volume distributed within a supply zone. This is defined as a geographically defined area, within which water intended for human consumption comes from one or more sources and within which water quality may be considered as being approximately uniform or within a given range. Under current domestic legislation, each PrWS has been designated as a water supply zone for the purposes of monitoring. This results in the frequencies of monitoring being applied to each individual supply. In 2016, LA records contained the details of a total of 36,565 private supplies in England, and 14,981 private supplies in Wales, of which 66% and 81% serve a single household respectively (DWI, 2017a, 2017b).
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